Tying in new addition CMU foundation wall to existing CMU foundation

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Old 01-11-13, 08:19 PM
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Tying in new addition CMU foundation wall to existing CMU foundation

I'm getting ready to build a new addition on our home. Existing house was built in 1965 on a 8x8x16 CMU crawlspace addition. I'm adding on addition onto the length of the house, the entire width using the same method, on a crawlspace. I have an excavator coming to dig and pour the footings, but after that it's all me (and several good friends). I consider myself an experienced DIY'er, and have tackled most of these renovations, new buildings, etc. in the past. Except this part.

See plans image of the area in question. Addition is 14'x37', 3' wider than existing house. The joists under the existing house run perpendicular to the joists in the addition. I'll be removing the exterior sheathing from the house and using that wall to hang drywall in new addition. There will only be a hallway passthrough at about the center of the house. The wood subfloor will be directly tied to the house with a band beam bolted on as shown.

Is there anything I need to do to tie in the new block foundation with the old foundation? Rebar, threaded rods, etc.? I can't really get to the inside cores of the existing foundation due to the sill plate. I assume all of the blocks are hollow core.



Any suggestions would be appreciated!
 
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Old 01-11-13, 09:46 PM
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It's not clear from your description if you will be building a new block wall immediately adjacent to the existing one, or just hope to carry the new TJI loads down through the existing wall. With either option, the existing footing (if marginal to begin with) at that location may be overstressed with the increase in loading. Although your local AHJ has probably approved whatever plans you've submitted to them for getting your permit. I'd be tempted to pour a new footing adjacent to and tied into the existing full-length footing, using No. 6s drilled and doweled in every 4' or so. Then tie new blocks to existing using standard spreader-cleats with one end flattened, alternating courses again at every 4' or so, drilled and mortared into the existing mortar joint openings. Doing so at the block interior web locations, of course, for maximum holding strength. You'd also gain more redundancy by tying in your new roof framing with the existing framing.
 
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Old 01-13-13, 07:22 PM
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Bridgeman makes a good point but I am not sure you will need the new footing and foundation at the existing wall. It depends on several factors that are not clear from your sketch. However, if the existing floor joist run parrallel to this existing wall then there is not a great deal of load on this wall presently (where the roof load goes is not clear). Assuming that the existing end CMU wall was constructed the same as the other existing walls I do believe you may be able to use the existing wall for bearing 1/2 of the new floor load. The floor load coming down on this wall would only be 400 to 600 pounds per lineal foot on the foundation which should not be an issue assuming you have a traditional spread footing under the existing wall.

Do you intend to bolt your (2) 2x10s to the existing rim joist and use joist hangers to carry the new TJIs or will you set the beam below the new joist and bolt the beam into the existing block wall? (that could be an issue if the cores are hollow)
 
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Old 01-25-13, 09:46 PM
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Thanks for the responses. For some reason I never received email notification, even though I had it set...

Sorry about the clarity in the drawing. The existing wall where the new addition is tying into is currently an exterior wall. The existing joists under the house do run parallel to this wall (up and down this webpage). So you are right, there is not a lot of load on this wall foundation. The new addition has the TJI joists running perpendicular to the existing joists. The double 2x10's will be thru-bolted to the existing rim joist and joist hangers will carry the new TJI's.

Where my question referred to was specifically at the top of the drawing (14' from the top left of the plan) and the similiar location on the bottom. The new CMU foundation wall will butt right up against this old CMU foundation wall. I don't see any good way to secure the new wall to the old. I have no access to the old wall as the sill plate is a 2x8 completely covering the holes in the block. So drilling holes into the old foundation and inserting re-bar won't really do any good. I could use epoxy, but there's not much surface area to grab to. And, I don't really think it is necessary though as the TJI joists will securely bind the new addition to the existing house foundation, helping prevent them from pulling apart...not that they ever would (both have foundations below the frost line)

I've recently talked to a neighbor/friend who built an addition on the back of his house and he did not tie the new block foundation to the existing either. This was done about 5 years ago, with no ill effects.
 
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Old 01-26-13, 06:04 AM
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I have a friend that used to own a house that had been added onto twice. The original house was quite small and had a full basement, then a 20' addition was added on the back with a crawlspace. Later on another 20' addition this time with a garage/workshop was added onto the previous addition. The lot was a lot lower in the back. I don't recall him ever having any foundation issues. The additions were decades old. The additions were noticeable at the roof and change of siding. He installed a new roof and vinyl siding which made it look like one continuous structure.

He would likely still be living there but he had septic issues and there was no room to add additional fill lines - he has a large family.
 
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Old 01-26-13, 06:18 AM
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Xref similar thread. If it gets too confusing or if we get too far along, I will merge the threads, so look at them both. http://www.doityourself.com/forum/ou...im-boards.html
 
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Old 01-30-13, 05:04 PM
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Sorry about the similiar thread issue; I thought this one specific to tying old and new foundations and didn't apply to that one...but I'll keep the conversion here.

So no need to worry about tying old to new foundation. Sounds good and what I had at first thought.

And good info on doubling up the rim boards on the two sides that run parallel to the I-joists. I do plan on using AdvanTech flooring. The I-joists will be hung from a double 2x10 band beam thru-bolted with 5/8" bolts @ 12" OC, staggered as spec'd by our architect. I'm wondering though...can I use double rim-boards to accomplish the same thing? I'm thinking they'd be more stable to go along with the otherwise engineered floor. But maybe rim boards can't be used in that manner... Feasible, or should I forget about it and stick with 2x10's?

Thanks,

Tom
 
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